Aromatic polymers have been previously hydrogenated using a variety of catalysts and conditions. Historically, typical hydrogenation catalysts have low reactivity, require high catalyst to polymer ratios and hydrogenate low molecular weight (less than 20,000) aromatic polymers. Hydrogenation of high molecular weight (Mw) aromatic polymers have also required high temperature and/or pressure conditions in order to achieve high hydrogenation levels. However, these conditions may cause polymer degradation.
Japanese Patent Application 03076706 describes a process for hydrogenating an aromatic polymer using a silica supported metal hydrogenation catalyst. These catalysts use a silica support of small pore diameter (200 to 500 angstroms), high surface area (100-500 m.sup.2 /g) and achieve hydrogenation levels of greater than 70 percent. However, to achieve high hydrogenation levels, large amounts of catalyst (1-100 weight percent based on resin) and high temperatures (170.degree. C.) are required which cause polymer degradation as exemplified by the decrease in the Mw after hydrogenation.
U.S. Pat. No. 5,028,665 describes a process for hydrogenating an unsaturated polymer using a supported metal hydrogenation catalyst wherein the support contains a majority of pores having diameters greater than 450 angstroms. However, the catalyst is limited by a small surface area and enables 90 to 100 percent olefinic hydrogenation but less than 25 percent aromatic hydrogenation.
U.S. Pat. No. 5,352,744 issued to Bates et al. describes a process for hydrogenating poly(alkenyl aromatic) or poly(alkenyl aromatic)/polydiene block copolymers, that provides hydrogenated polymers with 99.5% or greater saturation, using a metal catalyst on an alkaline metal salt support. Although Bates teaches from 0.01 to 10 grams of catalyst per gram of polymer may be used, a ratio of greater than 1.0 gram of catalyst per gram of polymer is needed to reach high hydrogenation levels.
Silica has long been used as a support for metal catalysts. Typically, the silica used as a support has had high surface area (200-600 m.sup.2 /g) and small average pore diameter (20 to 40 angstroms). Very low hydrogenation levels are obtained when hydrogenating high molecular weight aromatic polymers using metal hydrogenation catalysts supported by this type of silica.
Accordingly, it remains highly desirable to provide a process of hydrogenating an aromatic polymer at high levels which does not exhibit the foregoing disadvantages.